1. Field of the Invention
The present invention relates generally to a power supply apparatus for resistance welding, and more particularly, to a power supply apparatus designed to supply a controlled welding current via a switching element to workpieces from a capacitor storing welding energy as electric energy therein.
2. Description of the Related Arts
Up until now, such a resistance welding power supply apparatus has employed switching transistors as the switching element to provide a switching control of the switching transistors by pulse-width modulation (PWM).
The PWM is a technique for variably controlling the pulse width of an active level (e.g., high level) within each cycle at every fixed switching cycle defined by a clock. The PWM is used to effect a constant-current control allowing the welding current to conform to a set current value so that a switching control can be provided for correcting a deviation of the welding current relative to the set current value for each switching cycle.
The above transistor-based resistance welding power supply apparatus has often been applied to precision spot welding whose workpieces are mainly precise small-sized metal members such as electronic components. In such precision spot welding, desired welding energy must properly be supplied to the workpieces within an extremely brief weld time (e.g., 10 ms or below), which needs a finer and more rapid constant-current control.
It was however difficult for the conventional transistor-based resistance welding power supply apparatus using PWM to rapidly and finely correct the deviation or error upon the rise of current or upon the unexpected current variation in the constant-current control due to the fact that the switching control is restrained by the condition 0 less than Tw less than Tc for each switching cycle where Tc is a cycle of the switching cycle and Tw is a pulse width of the control pulse fed to the switching transistors for each switching cycle.
The present invention was conceived in view of the above problem involved in the prior art. It is therefore the object of the present invention to provide a resistance welding power supply apparatus capable of implementing a fine and rapid constant-current control and ensuring an improved resistance welding work quality.
In order to attain the above object, according to the aspect of the present invention there is provided a resistance welding power supply apparatus having a pair of welding electrodes through which a welding current flows, the pair of welding electrodes adapted to come into pressure contact with workpieces to effect a resistance welding on the workpieces, the power supply apparatus comprising a capacitor which stores electric energy for resistance welding in the form of electric charges; switching means for the supply of current electrically connected between the capacitor and one electrode of the pair of welding electrodes; and switching control means which, for the purpose of controlling the welding current during the supply of current for resistance welding, provide a control of the switching means such that, if the welding current is less than a set current value at a point of time of monitoring defined to occur at every predetermined time interval, then the switching control means keep the switching means in ON-state from the point of time of monitoring till the time when the welding current exceeds the set current value and that, if the welding current exceeds the set current value at the point of time of monitoring, then the switching control means keep the switching means in OFF-state till the next point of time of monitoring.
In the resistance welding power supply apparatus of the present invention, if the welding current is less than a set current value at a monitoring point defined at predetermined time intervals which is typically given by a clock, then the switching element is continuously kept in ON-state from that monitoring point till the time when the welding current exceeds the set current value without being affected by the clock cycle, whereas if the welding current exceeds the set current value at the monitoring point, then the switching element is continuously kept in OFF-state till the next monitoring point. Such a switching control allows the switching element to be turned on or off at an unfixed (non-cyclic) timing which is not restrained by the clock cycle to thereby correct (approximately to zero) the deviation or error of the welding current relative to the set current value in a minimum time and realize the rapid and fine constant-current control.